Methylmercury is an important contaminant of seafood and freshwater fish worldwide. Although tragic pollution episodes have demonstrated that the fetal brain is particularly susceptible to methylmercury toxicity, the upper limit for safe mercury exposure is unknown. A birth cohort of 1,000 children was formed during 1986 to 87 at the Faroe Islands, where increase exposure to methylmercury is mainly due to consumption of pilot whale meat. The fishing community is unique and highly suitable for population-based studies or prenatal methylmercury neurotoxicity: Average mercury exposures vary more than a 100-fold within the population, and socioeconomic factors and other confounding variables are of only limited concern. Ninety percent of the children from the cohort went through extensive neurobehavioral examinations at age 7 years, and the results showed mild deficits associated with prenatal exposures that were previously thought to be safe. These data will be scrutinized further statistically and neuropsychologically. In addition, to determine the long-term implications and the potential reversibility of mercury-associated deficits, follow-up of the cohort at age 14 years will be carried out. Neurobehavioral performance will be related to several mercury exposure biomarkers that reflect both prenatal and postnatal exposures. Exposures to polychlorinated biphenyls (PCBs) will also be assessed and analyzed for their possible neurobehavioral effects. Advanced statistical methods will be applied to provide documentation that can be used directly in risk assessment.